The goal of the proposed experiments in Project 2 of this competing renewal is to continue and extend studies on effects of gonadal steroid hormones, aging, and their interacfions, on the funcfion, neuroanatomy, and neurochemistry of the hypothalamic neuroendocrine network controlling reproducfion. We will address three sets of questions on the mechanisms by which the hypothalamus ages, and the neurobiological effects of hormones in the aging hypothalamus. Aim 1 will continue work started in years 6-10 on effects of aging and hormone treatment on the GnRH neural network in females. Aim 2 will inifiate a project to ascertain whether there is a critical window of opportunity and duration of treatment in order for effects of hormones to be manifested on hypothalamic gene and protein expression in females. The proposed experiments will utilize female rats given estradiol or vehicle replacement at different phases of their reproductive life cycle. A major priority of this project is to utilize hypothalami of female rhesus monkeys provided by Projects 3 & 4. As endpoints, we will quantify gene expression of and perform immunohistochemical studies to determine age- and hormonal changes to the hypothalamic GnRH circuits and their regulatory neurotransmitters (glutamate, GABA) and steroid hormone receptors. We will be able to differentiate effects of reproductive hormones from those of aging, to determine their interactions, and to draw comparisons between two important experimental species. Studies on hypothalamic aging are crifical to the goals of the PPG. Although the hypothalamus controls reproductive processes, its role in aging and hormonal regulation are not well understood. Thus, studies on the hypothalamus in Project 2 will serve as an important basis of comparison with the other PPG components. RELEVANCE (See instructions): Our studies are relevant to understanding normal neuroendocrine aging processes, and providing insights into new therapeutics for the treatment of neurobiological symptoms associated with the loss of hormones. We will learn whether there is a crifical period during which the hypothalamus may be responsive to hormone treatment, beyond which hormones are ineffective or may even be detrimental to functional outcomes.